The current down hole drill is a reciprocative impact tool that attaches to the end of a drill string. The down hole hammer drill consists of a piston moving up and down within a housing repeatedly striking a drill bit anvil. The down hole hammer drill has a reciprocating action and the repeated impact of the piston on the anvil face of a drill bit fractures the rock. The fast hammer action breaks rock into small flakes and dust and is blown clear by the exhaust from the down hole hammer drill. Drill pipes are added successively as the hole becomes deeper.
Pressurized fluid moves the piston up and down as directed by fluid passing through ports which are uncovered during the movement of the piston. Pressurized fluid enters the hammer through a bore in the back-head, opens a check valve and flows through channels to the lower chamber formed between the piston and the drill bit. As the pressure increases in the lower chamber, the piston is forced upward. The upward movement continues until the piston pulls off the stationary foot valve. This allows pressurized fluid to exhaust from the lower chamber through the hollow said foot valve and drill bit and into the borehole area being drilled. Immediately after the lower chamber is exhausted, a piston recess aligns with a port to allow fluid flow into the upper chamber. Exhaust passages in the upper chamber have been sealed by the piston and the pressure in the upper chamber increases. The increased pressure in the upper chamber causes the piston to decelerate, stop and then move in the reverse direction. As the downward movement starts, and just prior to the piston striking the drill bit, the upper chamber begins to exhaust.
Upon impact, the cycle is immediately repeated. The drill bit moves freely in the chuck splines so that impact force is transmitted to drill bit cutting elements to produce rock chips. When the hammer is lifted off the bottom, the drill bit drops to an extended position where it's top shoulder rests on a retainer ring. When the drill bit is resting on the retainer ring, the drill operator can flush water and debris from the hole and accelerate periodic borehole cleaning when required.
The reciprocation of the down hole hammer entails a foot valve that is rigid and fixed in the bore of the drill bit shank and extends from the anvil face. The piston slideably engages with the fixed foot valve on the downward impact stroke forming a sealed chamber causing the piston to decelerate until it impacts with the anvil face of the drill bit. The deceleration of the piston results in transferring a reduced impact force. This problem is seen as an unavoidable inefficiency as the increased pressure in the lower chamber is necessary in order to accelerate the piston upward to start a new cycle.
This problem has been identified and addressed by others. In U.S. 8006776B1 the issue of piston deceleration was addressed by introducing an asymmetrical timing valve that delays the onset of pressurized fluid supply to the lower chamber and extends the pressurization of the upper chamber during the power/impact stroke. Others including U.S. Pat. Nos. 5,984,021, 6,883,618 and 7,267,205 have described methods of controlling the duration of pressurized fluid supply, increasing pressures or using improved materials. Despite these attempts, the proposed solutions have not successfully addressed the problem of inefficiency due to piston deceleration.